1. Field of the Invention
The present invention relates to the structure of a semiconductor device used for electrooptic display devices such as liquid crystal display devices and organic EL (electroluminescence) display devices and other semiconductor components, and a method of manufacturing the same. In particular, the present invention relates to the structure of a semiconductor device including an aluminum alloy film (which is denoted by “Al alloy film” hereinafter), and either a Si film (silicon film) or a film containing Si as the main ingredient, as constituent elements of the semiconductor device, and a method of manufacturing the same.
2. Description of Related Art
An electrooptic display device with active matrix type TFTs in which thin film transistors (which are denoted by “TFTs” hereinafter) are used as the switching elements has been known as an example of semiconductor devices, and has increasingly found application as one of alternative flat panel display devices to the CRTs (Cathode Ray Tubes) in products in which the advantageous features of the electrooptic display devices such as low power consumption and low profile are fully exploited.
Conventionally, the so-called high melting point metal material such as titan (Ti), chromium (Cr), molybdenum (Mo), tantalum (Ta), tungsten (W), and alloys having these metals as the main ingredients has been used as typical material for wirings and electrodes that constitute a semiconductor device. These high melting point metals have almost no interfacial diffusion reaction in the bonding interface with the Si semiconductor film, and therefore have been suitably used as electrode material for semiconductor devices. However, as TVs have been becoming larger and the resolution of compact displays such as the displays of mobile phones has been becoming higher in recent years, the need to reduce the electrical resistance of wiring material has arisen. As a result, the specific resistance of high melting point metal (in general, 12 to 60 μΩ·cm) has no longer been considered to be sufficient. Consequently, aluminum (Al), which has low specific resistance and is easy to make wiring pattern, or Al alloy films composed of alloys containing Al as the main ingredients have become a focus of attention as the wiring material for display devices.
However, Al alloy films, in general, have been known to cause strong interdiffusion reactions in the bonding interface with a Si semiconductor film or a film containing Si as the main ingredient and thus to deteriorate electrical characteristics. Therefore, when an Al alloy film is to be connected to a Si film, the Al alloy film needs to be connected to it with above-mentioned high melting point metal interposed therebetween as a barrier layer. Furthermore, in the case of an optical display device for use in display apparatuses, wiring material (e.g., Al alloy film) needs to be connected to indium oxide group typically used as transparent pixel electrode material such as ITO (Indium Tin Oxide), which is prepared by combining indium oxide and tin oxide. As with the previous case, the Al alloy film also needs to be connected to the ITO with high melting point metal interposed therebetween as a barrier layer because the Al alloy film causes diffusion reactions in the interface with the ITO in such cases.
Examples of such structures in which above-mentioned high melting point metals are used as the barrier layers and applied to the source and drain electrodes of TFTs in combination with low-resistance Al alloy films are disclosed in Japanese Unexamined Patent Application Nos. 6-236893, 7-30118, and 8-62628. These examples have stacked-layer structures in which high melting point metals of Cr, Mo, Ti, and Zr are provided in the lower layer and connected directly to a low-resistance Si film to which impurities are added (ohmic contact Si film) and to an ITO film, and then low-resistance Al group metal is formed in the upper layer.
Meanwhile, Japanese Unexamined Patent Application Nos. 2003-89864 and 2004-214606 disclose methods for achieving better electrical characteristics in the interfaces (contact characteristics) by preventing interfacial diffusion reactions between Al alloy films and ITOs. These methods eliminate the need for the barrier layer of the high melting point metal at least in devices in which the Al alloy films need to be directly connected only with ITOs.
As described above, since interfacial diffusion reactions with Si semiconductor films and Si films containing Si as the main ingredients could not be prevented by the combination of conventional Al alloy films and manufacturing processes in the related art, the barrier layer of high melting point metal has had to be formed. As a result, the number of processes including a film-forming process and an etching process has increased, resulting in the decrease in the productivity. Furthermore, the difference in etching rates, the difference in side etching amounts in the lateral direction between the Al alloy film and the high melting point metal, and similar factors during the etching process have caused unevenness in the cross-sectional structure processed by the etching. As a result, micro-fabrication processes have been very difficult.
In addition, the unevenness in the cross-sectional structure processed by the etching has also deteriorated the coverage characteristic of the film formed in their upper layer. As stated above, Al alloy films and manufacturing methods in the related art have a problem that manufacturing semiconductor devices having high quality and high reliability has been very difficult and so forth.
Incidentally, the above-mentioned Japanese Unexamined Patent Application No. 2003-89864 also describes an improvement effect on the contact characteristic with Si in addition to the contact characteristic with ITO. According to evaluations conducted by the inventors of the present application in which Al alloy films were directly formed on Si semiconductors as the source and drain electrodes of TFTs using Si semiconductors, no interdiffusion reaction was observed in the interface with Si immediately after the film formation. However, the diffusion reactions had gradually proceeded with the heat treatment (it was maintained for about 30 minutes in the ambient atmosphere or nitrogen gas atmosphere), and they had proceeded to such extent at a temperature above 250° C. that the diffusion reactions could be observed even by an optical microscope or the like. Furthermore, although any remarkable diffusion reaction was not obverted by an optical microscope or the like at a temperature above 200° C., obvious deterioration in TFT characteristics, in particular, in a typical on/off characteristic of Id (drain current)−Vg (gate voltage) was observed by the measurements of TFT electrical characteristics. Typical manufacturing processes of active matrix TFT array substrates involve processes that are carried out, to say the least, at a temperature of 200° C. or higher. Therefore, there has been a problem that the application to such semiconductor devices has been practically impossible in terms of heat resistance.
One of the objects of the present invention is to provide an Al alloy film capable of achieving an excellent contact characteristic with a Si film or a film containing Si as the main ingredient without forming a barrier layer of high melting point metal, and a method of manufacturing the same.